Difficulty has been experienced in the past in producing high molecular weight polyesters from alicyclic or aromatic dicarboxylic anhydrides or their corresponding dicarboxylic acids (such as o-phthalic anhydride or o-phthalic acid). Much of this difficulty has been due to the fact that melt phase polymerization techniques have been used in which the ester groups tend to close up to revert back to the anhydride. Under the conditions of melt phase polymerization, the anhydride is volatile and vaporizes from the reaction system. Also, in the past, it has been common practice to use a molar excess of glycol. In polymerizations below 200.degree. C., imbalances in the molar amounts of acid and glycol components used in this invention limit molecular weight build-up. Thus, by the present invention whereby substantially equimolar amounts of acid and glycol components are used, and a solvent system is used rather than a melt phase system, high molecular weight polyesters from these acids and glycols can be obtained.
We are aware of no prior art which discloses the preparation of high molecular weight o-phthalate polyesters by the process of this invention. U.S. Pat. No. 3,079,368 discloses a melt-fusion method for preparing o-phthalate polyesters. U.S. Pat. No. 3,457,236 discloses an improvement to the melt-fusion method for preparing o-phthalate polyesters. Several glycols involved in this invention are mentioned in U.S. Pat. No. 3,457,236. The process of this invention is markedly different from the methods disclosed in these patents and offers clear advantages over them.
In U.S. Pat. No. 3,457,236, column 3, lines 65-75, and column 4, lines 1-6, equipment is described which includes a heated condenser and heated distillation head necessary to prevent clogging of the column with phthalic anhydride. With our process, this equipment is not required as no phthalic anhydride is lost, and a simple condenser and trap will suffice. Also, the method of this patent requires equipment for application of vacuum. While vacuum equipment is convenient for solvent removal in our process, it is not strictly required as the polyester may be recovered by other means. In column 3, lines 37-40, it is stated that an excess of glycol is required. Our process represents a savings in materials as equimolar amounts of reactants are used and no discard or recovery of excess monomer is required. The solvent used in our process is not lost as it is easily recovered and reused. In column 3, lines 66-69, it is stated that the temperatures required range from 170-280.degree. C. With our process, it is possible to prepare the polyester at a temperature as low as 110.degree. C. This fact and the lack of a need to heat condenser or still heads represent an opportunity to realize significant savings in energy costs. By utilizing our process over the method disclosed in U.S. Pat. No. 3,457,236 to prepare high molecular weight o-phthalate polyesters, considerable savings in equipment and energy and additional savings in monomeric materials can be realized.